Current methods of locating or tracking the position of a ship, submersibles, buoys or submerged instruments in the ocean can be broadly divided into two types. The first type employs electromagnetic transmission from a shore station, marker buoy or satellite. Such systems operate at long ranges and are limited in accuracy to about 100-200 m at best. Furthermore, these systems require above-surface antennas and are therefore of little use in positioning entirely submerged devices. The second type employs acoustic transmission from a set of transponders or beacons, usually moored to the ocean bottom, whose relative positions are precisely known.
The acoustic systems currently in use are pulse systems and continuous wave or Doppler systems. The pulse systems operate on the round trip transit times of tone bursts and are employed to compute the slant range from the object being tracked to the various underwater devices and based upon which the position of the object is determined. Currently, the pulse type of system is capable of determining object position with errors in the order of 2-3 m in 5 km water depth.
In the Doppler type of system or continuous wave system, the underwater devices continuously transmit acoustic tones of known frequency and the Doppler shift components of these tones as received at the object being tracked and these components are employed to determine the position of the object with respect to the bottom-moored reference net of underwater devices. Currently, systems operated in a continuous wave or Doppler mode can approach accuracies in the order of 3-4 cm.